namespace c++

Namespace std::cout or cout <<// A C++ program to demonstrate use of class 
// in a namespace 
#include <iostream> 
using namespace std; 
  
namespace ns 
{ 
    // Only declaring class here 
    class geek; 
} 
  
// Defining class outside 
class ns::geek 
{ 
public: 
    void display() 
    { 
        cout << "ns::geek::display()\n"; 
    } 
}; 
  
int main() 
{ 
    //Creating Object of geek Class 
    ns::geek obj; 
    obj.display(); 
    return 0; 
} 
// Creating namespaces 
#include <iostream> 
using namespace std; 
namespace ns1 
{ 
    int value()    { return 5; } 
} 
namespace ns2  
{ 
    const double x = 100; 
    double value() {  return 2*x; } 
} 
  
int main() 
{ 
    // Access value function within ns1 
    cout << ns1::value() << '\n';  
  
    // Access value function within ns2 
    cout << ns2::value() << '\n';  
  
    // Access variable x directly 
    cout << ns2::x << '\n';        
  
    return 0; 
}